In the conventional double beam analyzer with pneumatic detectors, one beam passes through the fluid sample to be analyzed and its attenuated because of absorption of infrared energy in a given spectral range by the presence of the component to be determined; the second beam of substantially equal intensity passes through a comparison fluid, normally substantially nonabsorbent in the measured spectral range of absorbence by the component to be determined. In Waters, U.S. Pat. No. 2,648,775, the two beams are modulated by an interrupter to separately and alternately project the two beams at a rapid frequency into a condenser microphone type detector responsive to the intensity of the beams in the spectral region of interest. The concentration of the fluid to be determined is indicated by the magnitude of the signal difference resulting from the two beams. Although the purpose of the high-frequency alternation was to avoid temperature effects between successive cycles, the instrument had no provision for correcting errors resulting from slower changes in ambient conditions and other instrument instabilities.